The present invention relates to a pressure sensor and more particularly to a sensor of the type comprising a rigid support and a diaphragm having a peripheral portion fixed by a layer of glue to the support and a central portion spaced from the support and carrying, on its surface facing the support, at least one thick-film resistor acting as a piezo-resistive transducer, the diaphragm being able to deform resiliently towards the support when a pressure is exerted on its other surface.
A pressure sensor of this type according to the prior art is shown in FIGS. 1 to 3 of the appended drawings. It includes, in known manner, a substantially circular rigid support 1, typically of ceramics material, having an annular projection 1a on its upper face. This projection surrounds a flat rebated surface 1b of the support (FIG. 2). The sensor further includes a diaphragm 2 constituted typically by a substantially circular disc of ceramics material having a thickness of the order of about 500 microns. This diaphragm has a peripheral portion fixed by a layer of glue 3 to the upper surface of the annular projection 1a of the support 1. Usually, both the diaphragm 2 and the projection 1a of the support 1 are provided with respective layers of vitreous glue deposited by silk screen printing, and the diaphragm and the support are then juxtaposed and passed through an oven so as to melt the layers of glue. Before the gluing step, there are deposited on the surface 2a of the diaphragm 2 intended to face the support 1, thick film resistors R for acting a piezo-resistive strain gauge transducers and conductive tracks (also deposited by silk screen printing by the thick-film technique) for their interconnection and their connection to external circuits.
In use, when a pressure is exerted on the diaphragm 2 in the manner indicated by the arrows indicated F in FIG. 3, the diaphragm 2 deforms elastically towards the support 1. The thick-film resistors R deform and consequently their resistances vary. This variation in resistance may be detected by external circuits of conventional type.
The pressure sensors of the type described above with reference to FIGS. 1 to 3 have the following problem. The dimensions of the diaphragm depend on the range of pressure values which are to be measured with the sensor. The diaphragm will be relatively thin for relatively low pressure ranges, while the diaphragm will generally have to be thicker to measure relatively higher pressures.
For some specific applications, it is necessary for the pressure sensor to be able to detect pressures within a predetermined range of measurements and also occasionally to be able to withstand very much higher pressures than the predetermined maximum pressure to be measured. For example, it may be necessary for a sensor to be able to measure pressures of between 0.5 and 2 bars, while, for safety reasons, it is necessary for the sensor to be able to withstand pressures of, for example, 10 bars. According to the prior art, the diaphragm of a pressure sensor intended to fulfil these requirements would be dimensioned so that it could withstand, that is, bend resiliently without breaking, a pressure of up to 10 bars. As a result of this dimensioning criterion, the sensor has a rather poor sensitivity in practice, that is, there is a relatively modest deformation of the diaphragm at predetermined pressure variations within the range of measurement proper, that is, for pressures of up to 5 bars.
The sensors of conventional type described above thus have the disadvantage of poor sensitivity in the range of pressure values to be measured when they have to be dimensioned so as to withstand occasional exceptional pressures of a much higher value than the maximum pressure to be measured.